Diffuser for sub-critical flow



Feb. 10, 1959 1 KARL-AXEL ZETTERSTRM DIFFUSER FOR SUB-CRITICAL FLOWFiled Sept. 16. 1955 Z}? mentor Ker/"A xe/ Z c wars tram of. theefficiency of the diffuser.

United States Patent ""ce 2,873,142 DIFFUSER FOR SUB-CRITICAL FLOWApplication September '16, 1955, Serial No. 534,859 2 to]. 299-141 Thepresent invention relates :to diffusers for subcritical flow and has forits object toprovide a diffuser the length of the diffuser channel ofwhich is considerably reduced at the cost of but a comparatively slightdecrease It is well known that in transforming the kinetic energy of agas current into pressure energy'by effecting a reduction of the speedof flow of the gas current and a corresponding increase of the pressurethereof, the channels required for carrying this transformation intoeffect must have a very great length owing to the centrifugal forcesappearing inside the gas mass. This, in its turn, often causes technicaldrawbacks, and the long channels may also give rise to large losses dueto friction against the walls of the channels, especially at high ratesof flow.

The reason for said comparatively long length of a standard type ofdiffuser is the action of centrifugal force and the resulting pressurevariation appearing inside the gas current owing to the mass inertia ofthe gas particles, as the gas current is caused to change its directionof flow so as to follow the wall surface of the diffuser. An additionalreason is the turbulence appearing in the layer of the gas immediatelyadjacent the diffuser wall.

If the diffuser pipe is too short with resulting too small radius ofcurvature of the surface of separation, a vortex will appear adjacentsaid surface. As a result, the paths of the particles of the regularlyflowing gas will deviate from the rigid wall surface so as to form asurface of discontinuity within the gas mass proper, thereby nulifyingthe diffusing action of the pipe.

If the flow along the limit surface of the rigid wall were regular andlamineferous altogether, considerably shorter radii of curvature couldbe used, resulting in a much shorter length of the diffuser pipe than isreally possible owing to the turbulence appearing adjacent the rigidwall surface.

The turbulence appears at every point of the surface of separation ofthe rigid wall, and upon the flow of the easily moving medium along saidsurface, a layer of turbulent medium the thickness of which increases inthe direction of flow, will thus continuously exist along said surface.The average rate of flow of such a turbulent layer is comparatively lowwith the result that, within such a layer only a very slight massinertia mayappear along the direction of flow, as long as said layerremains comparatively thin. According as the layer grows thicker it willbe influenced by pressure variations in a gradually increasing degree,thereby accelerating the particles of said layer in the direction ofpressure fall. Consequently, the turbulent medium will becorrespondingly displaced in said direction.

If the frictional forces appearing in the gas adjacent the wall surfaceof the diffuser are no longer sufficient to remove the turbulent medium,the layer thereof will rapidly grow thicker, thereby reducing the radiusof curvature of the regularly flowing medium so that said medium2,873,142 Patented Feb. 10, 1 95 9 loses its contact with the diffuserwall and the diffuser action gradually ceases.

In order to prevent to the largest extent possible the formation ofcavities along the wall surface of the diffuser it is, therefore, ofimportance to effect what may be termed a drainage of turbulent mediumfrom the wall surface. To this end the action of the frictional forceson that portion of the wall surface where the :turbulent layer isthickest should be increased in the highest degree possible and, inaddition, rthe difference in pressure appearing in the turbulent mediumalong the direction of flow within the same portion of the wall surfaceshould be reduced as far as possible. In respect to verylow rates offlow a comparatively shortdiffuser may be used without the formation ofcavities :at the wall surfaceof the diffuser pipe. If .in such a shortdiffuser the rate of .flow beincreased, cavities will be formed at thewall surface of the diffuser pipe. Where the formation of .cavitiesstarts, depends on, thecurvature of the wall surface along the length ofthe diffuser.

When, as is usually the case in respect to diffusion methods, it isdesired to obtain a flow including a rectilinear parallel movement withequalized speed of particles at the diffuser outlet, where the highestpressure prevails, all paths of current will have an inflection pointwhere the curvature of the paths of current changes its direction. Insuch a point the path of current and and the line of current coincide,the locus of the inflection points of all paths of current will thus bea level surface, that is to say, the same pressure exists at all pointsof this surface.

When it is desired with the aid of a partial diffusion at the wallsurface of the main diffuser to protect this surface against theformation of cavity, the wall surface at the smallest sectional area ofthe partial diffuser must change its direction of curvature, sinceotherwise cavity would appear in the partial diffuser. A partialdiffusion at the wall surface of the main diffuser may be obtained byplacing an annular shield inside the main diffuser which in conjunctionwith the wall surface of the main diffuser forms a partial diffuser.

By the provision according to this invention of a perforated shield atthe locus of the inflection points of the paths of current, where alsothe wall surface of the diffuser along the direction of flow changes itsradius of curvature, and by so shaping this shield at its periphery asto cause it to form a differential diffuser in conjunction with the wallof the main diffuser, a pressure increasing impulse action in adirection towards the wall surface of the main diffuser as well as apartial diffusion adjacent said wall surface may be obtained, with aresulting destroying of any cavity in the neighbourhood of the smallestcross section area of the partial diffuser.

In order to secure a reliable pressure and speed equalization at theoutlet end of the main diffuser, the perforated shield should be bodilysituated at or in the neighbourhood of the locus ofthe inflection pointsof the paths of flow of the particles.

The shortened diffuser according to the invention is considered tosatisfy in the best way the conditions above discussed. An embodiment ofsuch a diffuser is shown in the drawing in a diagrammatic longitudinalsection.

The diffuser illustrated in the drawing comprises a tube shaped memberthe wall 3 of which is widened at its outlet end as shown. Inserted in.the widened portion of the wall 3 is a perforated shield 1 in the shapeof a spherical cup having its convex surface facing the outlet of thediffuser. The peripheral, circular, usually imperforated portion 2 ofsaid perforated cup is curved towards said outlet in such a way as toform between itself and the surrounding widened end portion amen-1a ofthe wall 3 an annular passage 4 the cross section area of whichincreases in the direction of flow of the medium passing through thediffuser. Said direction is indicated by arrows in the drawing Theshield '1- as a whole is positioned in the neighbourhood of the levelsurface indicated'at 5 in the diffuser 3 which represents the locusof-the inflection points of the paths of flow of the particles of themedium. I claim:

' 1. A diffuser for sub-critical flow comprising a tubelike wall forminga main ditfuser' channel closed all around its circumference, said wallhaving an end'portion which is widened in cup-like form, a perforateddished shield disposed in said widened portion, said perforated dishedshield comprising a substantially spherically curved perforated centralportion and a nonfperforated annular circumferential portion curvedsubstantially in conformity with the curvature of the sur' roundingportion of the ditfuser wall though of a small radiusof curvature ascompared with that of said end portion or" said wall; said shield beingso positioned with relation to the surrounding portion of the main wallas to 4 divide the diffuser channel as a whole into a central passage ofa cross sectional area gradually increasing in the direction of flow andan annular passage of a cross sectional area also gradually increasingin the direction of flow so as to form a partial diffuser concentricallysurrounding said inner passage.

2. A diffuser as claimed in claim 1, having the further feature that thesubstantially spherically curved, perforated central portion of theshield is so positioned along the longitudinal direction of the difiuserwall as to generally coincide with the level surface inside the maindiffuser, channel at which the radii of curvature of the paths of flowof the particles of the medium passing through the diffuser "changetheir direction or become infinitely long.

References Cited in the file of this patent UNITED STATES PATENTS n ant

